Distributed Raman amplifier and optical communication system

Abstract

The distributed Raman amplifier monitors an OSNR of each channel in a WDM light which has been propagated through a transmission path to be Raman amplified, and thereafter, is amplified by an optical amplifier in an optical repeating node; judges whether a monitor value of the OSNR is larger or smaller than a previously set target value thereof; and feedback controls a driving state of a pumping light source which supplies a Raman pumping light to the transmission path, based on the judgment result. The optical communication system comprises the above distributed Raman amplifier in each repeating span thereof, and performs a pumping light control of the distributed Raman amplifier corresponding to the repeating span selected based on the OSNR in each distributed Raman amplifier and the monitor result of span loss. As a result, it becomes possible to effectively improve the OSNR of each channel in the WDM light, and also, to reduce the power consumption.

1. A distributed Raman amplifier for supplying a pumping light to a transmission path to Raman amplify a WDM light being propagated through the transmission path, comprising:
 a pumping light supplying section configured to supply a pumping light output from at least one pumping light source to the transmission path; 
 an OSNR monitoring section configured to monitor an OSNR representing a ratio between a signal light level of each channel contained in the WDM light which has been propagated through the transmission path to be Raman amplified and thereafter is amplified by an optical amplifier in an optical repeating node connected to a signal output end of the transmission path, and a noise light level of each channel contained in the WDM light; 
 a judging section configured to judge whether a monitor value of the OSNR of each channel in the OSNR monitoring section is larger or smaller than a previously set target value thereof; and 
 a control section configured to control a driving state of the pumping light source based on the judgment result in the judging section, 
 wherein the pumping light supplying section includes: 
 a plurality of pumping light sources of different wavelengths; and 
 a multiplexer configured to multiplex pumping lights output from the respective pumping light sources to supply the multiplexed light to the transmission path, and 
 the control section is configured to select only the pumping light source which corresponds to a pumping wavelength of which gain efficiency of Raman amplification is largest, among the plurality of pumping light sources, based on the judgment result in the judging section, and then to increase or decrease the output power of the selected pumping light source. 
 
     
     
       2. A distributed Raman amplifier according to  claim 1 ,
 wherein if it is judged in the judging section that the monitor value of the OSNR is smaller than the target value thereof, the control section is configured to increase the pumping light power to be output from the pumping light source, according to a deficiency of the monitor value of the OSNR relative to the target value thereof. 
 
     
     
       3. A distributed Raman amplifier according to  claim 2 ,
 wherein the control section includes: 
 a driving control circuit configured to control the driving state of the pumping light source so that a gain of the Raman amplifier in the transmission path is fixed at a previously set target gain; and 
 a control correction circuit configured to correct the target gain to be increased according to the deficiency of the monitor value of the OSNR relative to the target value thereof, to transmit the target gain after correction to the driving control circuit. 
 
     
     
       4. A distributed Raman amplifier according to  claim 2 ,
 wherein the control section includes: 
 a driving control circuit configured to control the driving state of the pumping light source so that optical output power per one channel of the WDM light propagated through the transmission path is fixed at a previously set target output level; and 
 a control correction circuit configured to correct the target output level to be increased according to the deficiency of the monitor value of the OSNR relative to the target value thereof, to transmit the target output level after correction to the driving control circuit. 
 
     
     
       5. A distributed Raman amplifier according to  claim 1 ,
 wherein if it is judged in the judging section that the monitor value of the OSNR is larger than the target value thereof, the control section is configured to decrease the pumping light power output from the pumping light source according to an excess of the monitor value of the OSNR relative to the target value thereof. 
 
     
     
       6. A distributed Raman amplifier according to  claim 5 ,
 wherein the control section includes: 
 a driving control circuit configured to control the driving state of the pumping light source so that optical output power per one channel of the WDM light propagated through the transmission path is fixed at a previously set target output level; and 
 a control correction circuit configured to correct the target output level to be decreased according to the excess of the monitor value of the OSNR relative to the target value thereof, to transmit the target output level after correction to the driving control circuit. 
 
     
     
       7. A distributed Raman amplifier for supplying a pumping light to a transmission path to Raman amplify a WDM light being propagated through the transmission path, comprising:
 a pumping light supplying section configured to supply a pumping light output from at least one pumping light source to the transmission path; 
 an OSNR monitoring section configured to monitor an OSNR representing a ratio between a signal light level of each channel contained in the WDM light which has been propagated through the transmission path to be Raman amplified and thereafter is amplified by an optical amplifier in an optical repeating node connected to a signal output end of the transmission path, and a noise light level of each channel contained in the WDM light; 
 a judging section configured to judge whether a monitor value of the OSNR of each channel in the OSNR monitoring section is larger or smaller than a previously set target value thereof; and 
 a control section configured to control a driving state of the pumping light source based on the judgment result in the judging section, 
 wherein the OSNR monitoring section includes: 
 a first branching device configured to branch a part of the WDM light amplified by the optical amplifier in the optical repeating node; 
 an optical filter configured to extract a noise component from the light branched by the first branching device; 
 a first optical receiving element configured to detect the power of the noise component extracted by the optical filter; 
 a second branching device configured to branch a part of the WDM light amplified by the optical amplifier in the optical repeating node, at a site different from the branching site by the first branching device; 
 a demultiplexer configured to demultiplex the light branched by the second branching device to takes out each channel; 
 a second optical receiving element configured to detect the power of each channel taken out by the demultiplexer; and 
 an OSNR computation circuit configured to compute the OSNR, based on the detection results in the first and second optical receiving elements and a route loss between the optical amplifier and the demultiplexer. 
 
     
     
       8. A distributed Raman amplifier according to  claim 7 ,
 wherein the second branching device is an OADM apparatus included in the optical repeating node; and 
 the second optical receiving element is an optical receiver which receives each channel demultiplexed by the demultiplexer. 
 
     
     
       9. A distributed Raman amplifier according to  claim 7 ,
 wherein the optical filter is a slanted fiber grating. 
 
     
     
       10. An optical communication system that transmits a WDM light from an optical transmitter to a transmission path, and repeating-transmits the WDM light to an optical receiver while amplifying the WDM light by at least one optical repeating node arranged on the transmission path; and also, is provided with a plurality of distributed Raman amplifiers each of which supplies a pumping light to the transmission path in each repeating span to Raman amplify the WDM light being propagated through the transmission path,
 wherein each of the plurality of distributed Raman amplifiers comprises: 
 a pumping light supplying section configured to supply a pumping light output from at least one pumping light source to the transmission path; 
 an OSNR monitoring section configured to monitor an OSNR representing a ratio between a signal light level of each channel contained in the WDM light which has been propagated through the transmission path to be Raman amplified and thereafter is amplified by an optical amplifier in the optical repeating node, and a noise light level of each channel contained in the WDM light; 
 a judging section configured to judge whether a monitor value of the OSNR of each channel in the OSNR monitoring section is larger or smaller than a previously set target value thereof; 
 a control section configured to control a driving state of the pumping light source based on the judgment result in the judging section; and 
 a span loss monitoring section configured to monitor a span loss indicating a loss in the transmission path to which the pumping light is supplied by the pumping light supplying section, and 
 the optical communication system further comprising: 
 a selecting section configured to select the repeating span in which the control of the pumping light source is preferentially performed from the plurality of repeating spans, based on the judging result in the judging section and the monitor result in the span loss monitoring section, which are transmitted from each distributed Raman amplifier; and 
 a control target value computing section configured, for the repeating span selected by the selecting section, to correct a control target value of the pumping light source according to an excessive or deficient amount of the monitor value of the OSNR relative to the target value thereof at a reception end, to notify the control target value after correction to the control section of the distributed Raman amplifier corresponding to the selected repeating span. 
 
     
     
       11. An optical communication system according to  claim 10 ,
 wherein the selecting section is configured to judge the channel in which the excessive or deficient amount of the monitor value of the OSNR relative to the target value thereof at the reception end becomes maximum, among each channel in the WDM light, and to select preferentially the repeating span of which span loss is relatively large, from the respective repeating spans on a transmission route of the judged channel. 
 
     
     
       12. An optical communication system according to  claim 11 ,
 wherein the selecting section is configured to select the repeating span of which span loss is maximum, from the respective repeating spans on the transmission route of the judged channel, and if the pumping light power in the distributed Raman amplifier corresponding to the selected repeating span reaches a predetermined value by the control by the control section, to select the repeating span of which span loss is the second-largest, from the respective repeating spans on the transmission route of the judged channel. 
 
     
     
       13. An optical communication system according to  claim 10 ,
 wherein when the transmission and reception of the WDM between the optical transmitter and the optical receiver is performed in point-to-point, 
 the OSNR monitoring section provided in each distributed Raman amplifier is configured to monitor the OSNR of one channel among each channel in the WDM light.